There are many different sources of VOC's that have fume concentrations that vary over time. For example, certain manufacturing batch processes will have VOC's fume flows that will vary from rich to lean over an extended period (such as every 24 hours).
In addition, soils and sludges contaminated with organic chemicals are a widespread problem throughout the world, with millions of cubic meters requiring remediation in the United States alone. For example, concentrated underground organic contaminant plumes are one of the most prevalent ground water contamination sources. A typical source of concentrated plume is a leaking underground storage tank. When the stored liquid escapes from the tank slowly, it can take years for the operator to become aware of the problem. By that time, the solvent or fuel can percolate deep into the earth, often into water-bearing regions. Collecting as a separate, liquid organic phase called Non-Aqueous-Phase Liquids ("NAPL's"), these contaminants provide a source that continuously compromises surrounding ground water. This type of spill is one of the most difficult environmental problems to remediate.
Clean-up of such contaminated materials is also subject to a wide variety of regulations in the United States, including those covered under The Comprehensive Environmental Response, Compensation, and Liability Act of 1980 ("CERCLA"), The Superfund Amendments and Reauthorization Act of 1986 ("SARA"), and The Resource Conservation and Recovery Act ("RCRA"). The total cost of these clean-up efforts in the United States has been estimated to exceed $200 billion over the next 30 to 40 years.
A number of processes can be used to deal with these problems of contaminated soils and sludges. Early techniques involved simply excavating the contaminated soil and disposing of it at a facility approved for the acceptance of hazardous waste. This method has high direct costs, and can lead to serious disruption of business operations for extended periods of time.
Attempts to remove such material by pumping the ground water required that huge amounts of water be washed through the system, requiring tens of years. Pumping at some sites for many years has resulted in effluent water that was apparently clean, but when the pumps were shut off and restarted several years later, the ground water again contained contaminates because they were not fully remediated in the first place.
Processes such as thermal desorption and in-situ soil vapor extraction ("SVE") allow for the controlled separation of organics from solids and soils. In these processes, the organic material is volatilized to produce an organic vapor, which thereafter must be removed or otherwise treated. This is in contrast to direct incineration, which involves heating solid material to destruction temperatures in -an oxidizing environment where volatilization and combustion of organics takes place simultaneously.
Soil Vapor Extraction
SVE is one current method for removing contaminants in situ. The SVE process is a technique for the removal of VOC's , and some semivolatile organic compounds ("SVOC's"), from the vadose zone. The vadose zone is the subsurface soil zone located between the land surface and the top of the water table. SVE creates a deliberate movement of air (or steam) through the soil by forcing a vacuum in a soil region, causing the organic compounds to vaporize and be removed with air through a system of wells to a vacuum system on the surface. The SVE approach is most suited to use after any free product or liquid has been recovered by conventional pumping techniques to remove occluded liquid remaining in the interstices of the soil particles.
In efforts to improve the efficiency of SVE remediation, supplemental techniques have recently been applied to standard SVE systems that include air sparging and steam injection. Air sparging allows for the recovery of the less volatile organics and dissolved contaminants and residuals beneath the water table by injection of heated air below the groundwater surface. The injected air enhances volatilization by increasing the water-to-air surface area and heating of the soil matrix. In some cases it may induce upward migration of globules of product with migrating air bubbles.
Steam injection injects steam into the contaminated zone to increase the subsurface temperatures, thereby volatilizing organic compounds with high boiling points. The added heat provided by the steam enhances the volatilization of organic residuals that are in the soil. The steam front mobilizes the heavy residuals and volatilizes the light fractions. Enhanced volatilization and residuals migration effects a faster, more complete mass transfer process that speeds the remediation and reduces cleanup costs.
SVE, whether or not combined with air sparging or steam injection techniques, must be used with other technologies in a treatment train since it transfers contaminants from soil and interstitial water to air and the entrained and condensed water waste streams--streams that require further treatment. Treatment of the contaminated air in typical SVE processes today includes either adsorption using activated carbon, condensation, or oxidation of the VOC's , catalytically or by incineration. Other methods, such as biological treatment, ultraviolet oxidation, and dispersion have also been used.
Carbon adsorption is the most commonly employed vapor treatment process and is adaptable to a wide range of VOC concentrations and flow rates. Skid-mounted, off-site-regenerated, carbon-canister systems are generally employed for low gas volumes and onsite-regenerated bed systems are employed for high gas volumes and cleanups of extended duration. Adsorption on granular activated carbon, however, is often unsuitable when the quantity of the contaminant is large, or the VOC's are not readily adsorbed because such situations lead to rapid saturation of the carbon. Furthermore, such systems only act to concentrate the vapors onto a solid bed that must be periodically backflushed to rejuvenate the carbon. Such backflushing raises the issue of contaminant disposal all over again.
Condensation can be used to separate the effluent VOC's from the carrier air. This is usually accomplished by refrigeration. The efficiency of this technique is determined by the effect of temperature on the vapor pressure of the VOC's present. Condensation is most efficient for high concentrations of vapors. The technology becomes less efficient as the clean up progresses and vapor concentrations drop. It may be ineffective during the last stages of the clean up. Since vapors are not completely condensed, a carbon adsorption or other additional treatment step may be required to remove residual vapors from the effluent stream.
Thermal destruction of contaminant vapors by incineration or catalytic oxidation can be effective for a wide range of compounds.
Catalytic oxidation is effective on hydrocarbon vapors. Recently-developed catalysts also permit the efficient destruction of halogenated compounds (bromides, chlorides, or fluorides). Nevertheless, although catalytic combustion or oxidation may be the preferred process, its use is constrained within certain limits. For example, if the concentration of the VOC's from a contaminant such as gasoline exceeds 25% of the lower explosive limit ("LEL"), the heat given off during oxidation raises the temperature of the catalytic oxidizer to a point of thermal deactivation of the catalyst. While such problems can, to a certain extent, be controlled by using dilution air ahead of the catalytic combustor, this means of temperature control is not practical at the high VOC concentrations often encountered during the early phase of a vapor extraction process.
Conventional flame-based combustion technologies, however, offer only adequate destruction efficiencies while generating secondary pollutants such as NO.sub.x. Other thermal oxidation systems, particularly those employing catalysts, have demonstrated that effectiveness is greatly diminished at elevated chlorinated hydrocarbon concentrations. Catalysis exhibits problems at chlorinated hydrocarbon concentrations of as low as 100 ppm.
Flame-based destruction processes also pose serious performance, regulatory, and public acceptance issues. Incineration is difficult to control and can result in the formation of highly undesirable by-products such as dioxins, furans, and oxides of nitrogen.
For example, standard combustors are particularly undesirable when dealing with chlorinated hydrocarbons. A free flame also results in incomplete combustion in some instances, and in uncontrollable production of undesirable side products. Because combustors typically operate at flame temperatures on the order of 3500.degree. F., significant amounts of unwanted NO.sub.x are often produced. Nitrous oxide (N.sub.2 O) and ammonia (NH.sub.3) are often by-products of NO.sub.x removal techniques. The high temperatures also raise significant safety issues.
Using current technologies, additional forms of contaminated residuals are typically produced from the application of SVE. These may include recovered condensate (contaminated water and possibly supernatant organics), spent activated carbon from off-gas treatment, nonrecovered contaminant in the soil, soil tailings from drilling, and air emissions after treatment. Each of these raise their own problems of disposal.
To date, the type of treatment chosen has generally depended on the composition and concentration of contaminants. For example, in cases where the concentration and/or the boiling point of the VOC's are low, condensation is economically impractical as compared to the capital and operating costs of adsorption or oxidation. The problem is that in many VOC abatement situations, the concentration of VOC's will vary over time. This is of particular concern in SVE, where a site may begin with a very high VOC concentration that, over several months of remediation, will drop off to a very low VOC concentration. For high VOC loadings, thermal oxidizers are typically preferred due to their ability to withstand high temperatures, and their low operating cost and high destruction rate. For low VOC loadings, catalytic oxidizers are typically preferred due to their lower operating temperature, which requires very little supplemental fuel addition. Conversely, when a thermal oxidizer is operated on low VOC fumes, high fuel consumption is required to maintain the high temperatures required by the oxidizer. When a catalytic oxidizer is operated on high VOC fumes, potential catalyst sintering and deactivation can result due to excessive temperatures.
Attempts have been made to overcome this operating cost/burnout scenerio by creating systems that utilize both a thermal oxidizer and a catalytic oxidizer, but such systems invariably have the two oxidizers as separate entities with some sort of switching mechanism for switching from one unit to another depending upon VOC concentration. One example of such a dual system is shown in U.S. Pat. No. 4,983,364 (Buck et al.). The very nature of such a dual, segregated system results in high capital costs.
Furthermore, while oxidation catalysts can be said to operate in a flameless, low NO.sub.x fashion, under most circumstances a pilot flame is used in a catalytic oxidizer in order to maintain catalyst temperature at an optimum level. In part, this is due to the fact that the normally available supplemental fuels, such as natural gas and propane, consist of low carbon species that will not readily oxidize in most catalysts. Additionally, because of the typically high cost of the catalyst itself, most catalytic beds are quite small and the possibility of a fume concentration variation disrupting the steady state nature of any catalytic bed operated without the use of a supplemental pilot flame weighs in favor of using a pilot flame. Consequently, temperature control of the catalyst is achieved using direct flow-through of hot combustion off-gases from an upstream pilot flame. The problem with this method, however, is that, while the catalytic oxidation itself is inherently a low NO.sub.x process, the diffusion pilot flame puts significant levels of NO.sub.x into the catalytic oxidizer exhaust gas.
Thermal Desorption
Thermal desorption has been successfully demonstrated for the treatment of soils and solids contaminated by organic compounds. Treatment of soils contaminated with organic compounds, dioxin, polynuclear aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and low level mixed wastes using thermal desorption is known. Recognizing the cost-competitive nature of thermal desorption, many remediation companies have diversified their capabilities to include thermal desorber processes. Others are converting existing fluidized bed incinerators to thermal desorbers emphasizing thermal desorption as the preferred thermal treatment process. In addition, thermal desorption has proven to be an effective method in remediating mixed wastes or soils contaminated with both radioactive compounds and organics.
With thermal desorption, the process removes organic contaminants by indirectly heating the soils and solids to temperatures sufficient to vaporize the hazardous components. The soil is typically heated to no higher than 550.degree. C., and frequently the heating occurs in the absence of oxygen.
The thermal desorber, acting as a separator, removes the organic contamination, leaving a residue that contains inerts, radioactive material (when present), and metals in the soil. Once the treated soil has been stabilized to prevent any metal salts in the soil from dissolving in water, the stabilized material can be characterized and disposed of or handled as a low-level radioactive waste.
After volatilization, the organic vapors in the off-gas are presently typically treated by either oxidation in a high temperature combustion chamber/incinerator or by condensation and conventional treatment of the small amount of resultant condensate such as by capturing on a carbon substrate. Examples of known systems for thermal desorption that use a following combustion technique include those described in U.S. Pat. No. 5,282,695 (Crosby et al.), U.S. Pat. No. 5,228,803 (Crosby et al.), U.S. Pat. No. 4,974,528 (Barcell), U.S. Pat. No. 4,961,391 (Mak et al.), U.S. Pat No. 4,925,389 (DeCicco et al.), U.S. Pat. No. 4,815,398 (Keating et al.), U.S. Pat. No. 4,766,822 (DeCicco et al.), and U.S. Pat No. 4,746,290 (DeCicco et al.). Examples of known systems for thermal desorption that use a following condensation technique include those described. in U.S. Pat. No. 5,098,481 (Monlux), and U.S. Pat. No. 5,228,803 (Crosby et al.).
Thermal desorption techniques have problems, however, particularly in the need for further processing after the contaminants are volatilized. Examples of problems that arise when condensation is used for post-volatilization treatment include disposal issues surrounding both the carbon used for adsorption and the recovered liquid organic wastes. As such, the direct destruction of waste organics into benign products; such as water, carbon dioxide, and salts is frequently preferable as a final solution.
The use of destruction technologies in the processing of volatilized contaminants typically involves the thermo-chemical reformation of the organic compounds into such oxidized products. While this is desirable as a final solution, as discussed previously with respect to SVE, flame-based destruction process can pose serious performance, regulatory, and public acceptance issues.
The difficulties and expense of obtaining operating permits for hazardous waste treatment processes utilizing flame based technologies, either for direct soil incineration or for incineration of volatilized contaminants, is also well known. Alternatively, the encumbrances of dealing with contaminated carbon wastes or off-site liquid waste disposal inherent with the condensing option increases the costs of that type of system and affects operational factors negatively. Furthermore, the relatively low temperatures associated with thermal separation can be the optimum temperature for converting PAHs and chlorinated phenolics into dibenzo furans and dioxins.
Thus, it can be seen that there is a need for a practical means of destroying organics removed from contaminated soils or other sources of variable concentration VOC's that avoids the various difficulties and inefficiencies of the prior art. There is a further need for such a system to result in high destruction and removal efficiency ("DRE") of the organics in a cost-effective manner.